1. Field of the Invention
The present invention relates to a power plant for driving a driven part, and more particularly to a power plant including a prime mover and a rotary motor as power sources.
2. Description of the Related Art
Conventionally, as the power plant of this kind, one disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2004-175320 is known. This power plant is for driving drive wheels of a vehicle, and is equipped with an internal combustion engine and a motor generator as power sources, and a stepless transmission and a planetary gear train for transmitting power to the drive wheels. The planetary gear train is a general type that has a sun gear, a ring gear, and a carrier, and the sun gear and the ring gear are connected to the drive wheels via a high clutch and a low clutch, respectively. The engine is connected to the carrier via a main shaft, and the main shaft is connected to an input pulley of the stepless transmission. Further, the motor generator, the output pulley of the stepless transmission, and the sun gear are connected to each other via an auxiliary shaft.
The power plant configured as above has operation modes one of which is a torque circulation mode mainly used during low-speed traveling. In the torque circulation mode, the aforementioned high clutch is disengaged to thereby disconnect between the sun gear and the drive wheels, and the low clutch is engaged to connect between the ring gear and the drive wheels. Thus, the torque of the motor generator is transmitted to the carrier via the auxiliary shaft, the stepless transmission, and the main shaft. Further, the torque transmitted to the carrier is distributed between the ring gear and the sun gear, and torque distributed to the ring gear is transmitted to the drive wheels. On the other hand, torque distributed to the sun gear is transmitted to the auxiliary shaft, and after being combined with the torque of the motor generator, it is transmitted to the carrier via the stepless transmission and the main shaft. As described above, in the torque circulation mode, the power of the motor generator is transmitted to the drive wheels while circulating through the stepless transmission and the planetary gear train.
As described above, in the conventional power plant, since during operation in the torque circulation mode, the transmission of power from the motor generator to the drive wheels is carried out necessarily via the stepless transmission, and hence due to power transmission loss at the stepless transmission, the drive efficiency of the power plant is reduced. Further, for example, during the operation in the torque circulation mode, also when electric power is generated by the motor generator using the power of the drive wheels, the transmission of power from the drive wheels to the motor generator is performed via the stepless transmission, and hence it is also impossible to obtain a sufficient power generation efficiency due to power transmission loss at the stepless transmission.